Hi Chris
Sorry to hear your in the doldrums or wilderness as you put it,seems to me like you've started to beat yourself up about how good you feel you are and don't think your getting any better at one or two things no matter how hard you try.In truth you're a far better player than you give yourself cedit for.This sight reading speed thing that seems to bother you,one thought is do you wear glasses for reading,I do and although I can still read the music without them I find using makes it that much easier,maybe if your not using reading glasses it may be worth an eye test,"only a thought"
.This sight reading ability is one of the reasons I haven't dived into playing with a band because I know if someone just threw a score at me and said lets go I wouldn't be able to do it,now I'm sure that, as you said the band has a very limited tenor section and knowing what a good player you are it would be worthwhile for him to make sure you had the score in advance to give you time to get up to speed with it,may be worth a word in his ear.
You seem to be worrying about how fast you can play a piece and I can't imagine why,playing fast isn't the be all and end all of everything It's far harder to play something slow and tonely beutiful than it is to thrash through something and as my tutor says to me the key to fast is SLOW and repetition and he means REALLY REALLY SLOW but with the correct flow of the music,whereby your fingers get automatic memory of the bit your playing and it stays there when you speed up,I personally don't think speed is all that important unless you want to be some sort of jazz soloing wizard.
As for what to do to get your mojo back I would say go back to the pieces you know really well and try playing them differently and perhaps recording these variations to see how you personally like the variations,it's surprising how much fun you can have doing this.
Thanks for all your help in the past I hope in some small way this helps you...john